Recently, further downsizing of an apparatus, resource saving in response to ecology and cost reduction have been desired for electrophotographic printers and copiers.
Methods to solve these problems include lowering the fixing temperature and there have been attempted, as an achieving means therefore, a lowering of the molecular weight of a binder resin constituting a toner, depression of glass transition point (Tg) and increasing the content of a wax contained in a toner.
However, a molecular weight lowering or glass transition point depression of a binder resin leads to a depression of the melting temperature but results in deteriorated storage stability of the toner, and specifically under an environment of high temperature, cohesion onto a developing device or fusion of toner particles results and leading to a lowering of fluidity. As a result, the initial rise of electrostatic charging is lowered, resulting in scattering of the toner or occurrence of fogging.
There have been made some proposals to overcome these problems. For instance, there was disclosed a technique of adding organic particles of 50-200 nm to toner particles to effectuate a spacer function, as disclosed in JP 06-266152A. In that case, the use of spherical organic particles enables to effectuate a spacer function in the early stage. However, such organic particles are rare in burial or liberation due to stress with aging but the particles themselves deform, rendering it difficult to stably maintain enhanced spacer function over a long period of time.
On the other hand, JP 07-261446A disclose a technique of adding a large-particulate silica in addition to small-particulate silica, as a fluidizing agent. According to the disclosure, adhesion between toner particles was prevented by a spacer effect of the large-particulate silica, thereby inhibiting fusion of toner particles and restraining fogging. However, it was proved that a large-particulate silica of more than 100 nm caused scattering of toner particles or resulted in a lowering of fixability or image defects such as white spots or white streaks. This is supposed to be due to the fact that silica, being heavy in true specific gravity, results in increased liberation from toner particles along with increased particle size.
To overcome these problems, there were proposed silica capsule particles in which resin particles were covered with a silica layer through a sol-gel method, as disclosed in JP 2005-173480A. Silica capsule particles obtained by the sol-gel method exhibit a relatively low specific gravity and satisfactory initial performance of a toner, such as fluidity, electrostatic chargeability, developability, transferability and fixability can be achieved by subjecting them to an external-addition treatment. However, there were produced problems such that silica layer coverage was stripped after being used over a long duration, resulting in deteriorated fluidity and rendering it difficult to attain the desired image density. Specifically, deterioration in durability was marked when the particle size of an external additive became 80 nm or more.